In the semiconductor manufacturing industry, it is necessary to achieve precise control of the quantity, temperature and pressure of one or more reactant materials which are delivered in a gaseous state to a reaction chamber. Mass flow controllers (MFC) are widely used in the semiconductor manufacturing industry to control the delivery of process reactants.
A typical MFC generally includes a main conduit including an upstream portion connected to an inlet of the MFC and a downstream portion connected to an outlet of the MFC. The MFC also includes a mass flow rate sensor for measuring the rate of flow of gas through the MFC, a valve for controlling the flow of gas through the MFC and a simple control circuit or a computer mounted on a P.C. board and connected to the mass flow rate sensor and the valve. The computer, or processor, is programmed with a desired flow rate through a connector, for example, which the computer compares to an actual flow rate as measured by the mass flow rate sensor. If the actual flow rate does not equal the desired flow rate, the processor is further programmed to open or close the valve until the actual flow rate equals the desired flow rate.
Thermal mass flow sensors operate on the principle of conservation of thermal energy, where power applied to a flowing gas equals the mass flow rate of the gas multiplied by the specific heat of the gas, the density of the gas and the temperature change of the gas. The mass flow rate can therefore be determined if the properties of the gas, the temperature changes of the gas, and the rate of power applied to the gas are known.
The thermal mass flow rate sensor includes a sensor tube and a bypass tube connecting the upstream portion of the main conduit to the downstream portion of the main conduit such that flow through the main conduit is divided through the sensor tube and the bypass tube. The sensor employs the sensor tube as the primary sensing mechanism. Typically the sensor tube is significantly smaller than the primary conduit. A laminar flow element is normally placed in the bypass tube to provide laminar flow in the bypass tube for a predetermined range of flow.
The thermal mass flow rate sensor also includes one or more heating elements attached to the sensor tube to allow a heat transfer from the heating elements, through the tube and to the gas. The heating elements also serve as resistance temperature sensors that track the local temperature of the wall of the sensor tube. Heat transfer between the gas flowing in the sensor tube from the tube walls is a function of the difference between the gas temperature and the wall temperature, and the heat transfer rate coefficient inside of the tube. The increase in gas temperature between the two heating elements is a function of the mass flow rate of the gas through the sensor tube, the specific heat of the gas, and the power delivered to the heater elements. A circuit converts the difference in resistance (or temperature) of the two elements into a voltage output (power) which is calibrated to known flow rates. Normally, the change in resistance is converted to voltage by a Wheatstone bridge, which is connected to the processor. The processor compares the voltage level to stored reference gas calibration data to determine the flow rate. The stored reference gas calibration data, or table, includes voltages produced by the sensor for a range of known flow rates of the reference gas.
Since the calibration data changes for gases other than the reference gas, a characterization of the calibration data is required for each type of gas being measured in the thermal based mass flow rate sensor, in order for the resulting measurement to be accurate. This characterization is also referred to as multi-gas correction functions. The multi-gas correction function is the ratio of flows, in the sensor tube only, of the new gas over the reference gas (Qnew/Qref). This ratio changes with sensor voltage. The calibration table of the reference gas is simply a list of sensor voltages and measured total flows at those voltages. To obtain the calibration table in the new gas, the flow of the reference gas is multiplied by the multi-gas correction function at each voltage in the reference gas calibration table.
The multi-gas correction function assumes that a bypass ratio is the same in both the reference gas and the gas being measured. The bypass ratio η (also referred to as split ratio) of the sensor is given by the ratio of the total amount of gas flowing through the bypass tube and the sensor tube divided by the amount of gas flowing through the sensor tube only. However, the bypass ratio changes for different gases because of pressure losses (i.e., bypass ratio error), such as entrance effects, caused by non-ideal geometric conditions of the primary conduit, the bypass tube and the sensor tube. These pressure losses are often referred to as “Reynolds Losses” because the losses are a function of the Reynolds number of the gas being measured. The Reynolds Losses can be a major source of error in measuring the gas flow.
One method of compensating for the Reynolds Losses such that the bypass ratio is the same for all gases is to actually calibrate the sensor, including both the sensor tube and the bypass tube, for all gases at know flow rates, and provide another calibration table for each gas. However, this is an expensive and time consuming solution. Another method of compensating for the Reynolds Losses is to limit the sensor to low flow rates so that the multigas correction function reduces to a single coefficient. A further method of compensating for the Reynolds Losses such that the bypass ratio is the same for all gases is to provide the bypass tube, and/or a laminar flow element located within the bypass tube, with a relatively great length such that the entrance effects are made negligible. This method, however, prevents a flow sensor having a compact design.
It is an object of the present disclosure to provide a new and improved thermal mass flow rate sensor which can be used with different gases. Preferably, the new and improved thermal mass flow rate sensor will provide compensation for Reynolds Losses between different gases.